While radioactive waste water, i.e. waste water containing radioactive nuclides or radionuclides, can result from any industrial applications of nuclear energy, they are most frequently obtained as the sump product of nuclear electric-power generating stations.
To prevent dispersal of radioactivity into the environment, the radioactive waste water is subjected to treatment, thereby increasing the concentration of the resulting product in radionuclides and reducing the volume of the waste which must be stored or handled.
A typical approach to the decontamination of radionuclide recovery and disposal of such contaminated waste water is to subject the same to evaporation.
In prior-art systems, the evaporation is carried out by introducing the radioactive waste water into a circulation evaporator which transforms the water into a vapor phase. The radionuclides, which are in the form of impurities in the water, are transformed during the vaporization step into liquid or solid droplets or particles, customarily dispersed as an aerosol in the vapor phase.
The gaseous effluent from the evaporation unit is treated to remove this aerosol therefrom. For example, a separator column is usually provided downstream of the evaporator and may be constructed as or with a mist breaker, mist "collection" or other conventional mist-removal unit. For instance, it might be constituted as a bubble-plate column, a screen-plate column or a packed column containing a multiplicity of filter bodies.
In addition to the separator, demisters or mist breakers can be provided, e.g. in the form of thick wire mesh of steel or synthetic resin as are commonly used in the chemical arts in dealing with aerosol separation.
In the treatment of radioactive nuclear waste one should distinguish between the so-called system-decontamination factor (DF.sub.s) and the vapor decontamination factor (DF.sub.v).
The system decontamination factor DF.sub.s is the ratio of the specific activity of the waste water to that of the distillate and is of a value or quantity which is significant in dealings with respective control authorities such as governmental units. The evaporator decontamination factor DF.sub.v establishes a ratio between the specific activities of the concentrate and the distillate and can correspond approximately to 100 times (10.sup.2) the system decontamination factor DF.sub.s. The evaporator decontamination factor DF.sub.v is established by the parameters of the evaporator unit and hence by the manufacturer. Practical experience with known decontamination systems has shown that the assumed DF.sub.s values of 10.sup.8 to 10.sup.12 are not attained in operation but that, at best, DF.sub.s values of 10.sup.4 to 10.sup.6 can be obtained. The actual DF.sub.s values, moreover, decrease with duration of operation of the system.
Considerable investigation has shown that a primary reason for the low DF.sub.s values actually obtained is the continuous contamination of the fittings of the separator column because the particles to be separated from the vapor tend to deposit on the fittings or to promote deposition on surfaces thereof.
For high DF.sub.s values, therefore, efforts have been made to effect cyclical cleaning of the fittings, i.e. cleaning of the fittings after the lapse of a predetermined operating period, or replacement of the fittings. Both of these operations are expensive and time consuming, necessitating shutdown of the evaporation installation for long periods and to possible exposure of operating personnel to high radiation dosages.